1. Technical Field
The present invention relates to fluid-dynamic-pressure bearings and spindle motors furnished with the fluid-dynamic-pressure bearings, to methods of manufacturing rotor assemblies applied in the spindle motors, and to recording-disk drives furnished with the spindle motors.
2. Background Art
As bearings for motors that drive recording disks in hard disk drives, removable disk drives and similar devices, to date fluid-dynamic-pressure bearings have been employed and a variety have been proposed. Fluid-dynamic-pressure bearings exploit dynamic pressure generated, when the motor spins, in a lubricating fluid such as oil retained in a gap in between, as for example shown in FIG. 6, a shaft 102 and a sleeve 104.
In a conventional motor such as is depicted in FIG. 6, a pair of radial bearing sections 106 provided at an axial separation is formed in the gap in between the cylindrical outer surface of the shaft 102 and the cylindrical inner surface of the sleeve 104, and top and bottom thrust bearing sections 112 and 113 are formed in respective gaps between upper and lower faces of the rim of a thrust plate 108 fixed unitarily onto the shaft 102 and, opposing these faces, a lower surface of the sleeve 104 and an upper surface of a counterplate 110.
A further feature involving the cylindrical inner surface of the sleeve 104 and the cylindrical outer surface of the shaft 102 in this conventional motor is a capillary seal 118 formed in the interval between the radial bearing sections 106 and the snugged-fit section 116 between the shaft 102 and a rotor hub 114 fixed to the upper portion of the shaft 102. The cylindrical outer surface of the shaft 102 is constricted gradually, parting away from the radial bearings 106 as a pair, to form the capillary seal 118. Depending on the position where the gas-fluid interface forms in the oil retained within the capillary seal 118, a differential in capillary force will be produced in the capillary seal 118; and if the amount of oil that is retained by the radial bearing sections 106 and the top and bottom thrust bearing sections 112, 113 has decreased, oil is supplied from the capillary seal 118 to the radial bearing sections 106 and the top and bottom thrust bearing sections 112, 113. Likewise, if the volume of oil retained within the radial bearing sections 106 and the top and bottom thrust bearing sections 112, 113 has increased due to spindle-motor temperature elevation accompanying motor rotation, then that increase is accommodated.
In this way oil is continuously, without interruption retained in the micro-gap that forms the radial bearing sections 106, the top and bottom thrust bearing sections 112 and 113, and the capillary seal 118. (Such an oil-retention structure will be denoted a “full-fill structure” hereinafter.) When the motor spins, in the radial bearing sections 106 and the top and bottom thrust bearing sections 112, 113 dynamic pressure is generated, through which the sleeve 104 supports the shaft 102 and the rotor hub 114 in a non-contact bearing that lets them spin.
In recent years recording-disk drives that had been employed in personal computers and like devices have begun to be applied in information terminals further scaled-down for carrying along on the go, which has led to the desire for the spindle motors to be further downsized, slimmer profile, and lower power consuming, in addition to the high-speed and high-precision rotation traditionally expected from the spindle motors.
Nevertheless, if the spindle motor is to be made smaller-sized and vertically slimmer, the fact that the construction described above configures the snugged-fit section 116, the capillary seal 118, and the pair of radial bearing sections 106 and the top and bottom thrust bearing sections 112, 113 ranged in a line axially is prohibitive of scaling down and slimming down the spindle motor.
Put differently, against demands for miniaturized, slimmer spindle motors, maintaining the axial span necessary between the pair of radial bearing sections 106 to ensure sufficient bearing stiffness would stand in the way of maintaining the axial dimension that the snugged-fit section 116 and the top and bottom thrust bearing sections 112, 113 require. Shortening the axial dimension of the snugged-fit section 116 would weaken the clamping strength between the shaft 102 and the rotor hub 114, which would lead to the rotor hub 114 losing levelness when the motor is spinning, with the rotor hub 114 wobbling such that stabilized rotation could never be gained.
On the other hand, attempting to maintain the axial dimension that the snugged-fit section 116 requires would shorten the axial dimension of the pair of radial bearing sections 106, which would weaken the radial bearing stiffness such that the bearings could not stably support the shaft 102. The fact that maintaining the rotational precision and the attitude of the shaft 102 and rotor hub 114 depends exclusively on the pair of radial bearing sections 106 requires that sufficient axial span between the pair of radial bearing sections 106 be available. Consequently, scaling down and reducing the profile of a spindle motor as described earlier while sustaining the rotational precision called for in the motor proves to be extraordinarily challenging.
What is more, attempting to maintain the axial dimension that the pair of radial bearing sections 106 as well as the snugged-fit section 116 require is prohibitive of ensuring requisite bearing stiffness in the top and bottom thrust bearing sections 112, 113. In the conventional motor under discussion, the thrust plate 108 is fixed unitarily to the end portion of the shaft 102, wherein the axially directed load-bearing force generated by the top and bottom thrust bearing sections 112, 113 formed on the upper and lower faces of the rim of the thrust plate 108 governs the axial travel of the shaft 102 and rotor hub 114, stabilizing the lift on the shaft 102 and rotor hub 114.
Given the circumstances, then, making the axial dimension of the thrust plate 108 thinner in an attempt to trim the axial extent of the top and bottom thrust bearing sections 112, 113 would preclude attaining stabilized, axially directed load-bearing force in the thrust bearing sections 112, 113, compromising the bearing stiffness in those bearing sections. Such problems as over-lift on the shaft 102 and rotor hub 114 would occur as a consequence, which would be prohibitive of stably supporting the shaft 102 and rotor hub 114.
Another application of recording-disk drives that has begun of late is the installation of the drives in vehicle on-board devices, typified by car navigation systems. Yet in implementations in vehicle on-board devices, since the recording-disk drives are expected to perform under various environments, stable operation within an extremely broad temperature range is being demanded of the recording-disk drives. Use under severe temperature environments that recording-disk drives have not met with until now—for example, use under environments where changes in temperature that range across 100° C. or more are a possibility—is being called for.
The fact that, as is well known, the viscosity of oil drops under high-temperature environments means that the dynamic pressure generated by oil-filled dynamic-pressure bearings in such environments also falls, which consequently is prohibitive of attaining predetermined bearing stiffness. Employing a highly viscous oil in order to avert such degradation in oil viscosity means that the oil will be excessively viscous under low-temperature environments, increasing the rotational load on the motor, such that ultimately the amount of power that the motor consumes will grow. Consequently, in order to make broad-temperature-ranging application of a motor using a fluid-dynamic-pressure bearing possible, problems that run counter to each other—under low-temperature environments restraining increase in power consumption by the motor, while under high-temperature environments preventing degradation in bearing stiffness—must be resolved at once. Moreover, under high-temperature environments, along with the oil viscosity becoming less viscous, the volume of the oil increases due to thermal expansion. As a consequence, of the oil retained in the fluid-dynamic-pressure bearing sections, that portion by which the oil has volumetrically increased is forced out from the bearing sections into the capillary seal 118. Under those circumstances, if owing to the dimensional constraints of miniaturizing and slimming down the motor, the axial dimension of the capillary seal 118 is limited such that sufficient capacity for the seal cannot be secured, there would be occasions when oil flowing into the capillary seal 118 is not taken up completely, such that the oil would flow out to the exterior of the capillary seal 118. If escaped oil adheres to the hard disks in the disk-drive area, or to the magnetic heads arranged in close proximity to the disks, the oil will become a cause that gives rise to read/write errors.
Against this backdrop, attempting to secure sufficient axial extent for the capillary seal 118 to retain that portion by which the oil has volumetrically increased as just described would constrain the axial dimension of the pair of radial bearing sections 106 ranged axially in line with the capillary seal 118, which would prove prohibitive of ensuring requisite bearing stiffness in the radial bearing sections 106. Moreover, securing the axial extent that the snugged-fit section 116 between the shaft 102 and rotor hub 114, which is likewise ranged axially in line with the capillary seal 118, requires would also prove to be problematic.
A further consideration in designing miniature, slim spindle motors is that thus scaling the motors entails as a matter of course that the various parts constituting the motor are also miniaturized and reduced-profile. This means that the mechanical strength of the various parts is that much the weaker, and thus the influence that manufacturing stresses, occurring in processes such as pressure-fitting or bonding the parts together, have on the surface precision of and distortion in the parts proves to be considerable.
For example, when a rotor magnet 120 is to be adhesively fastened to the rotor hub 114, because the rotor magnet 120 is not a very high-strength component, as a means for fixing the two, the rotor magnet 120 cannot be snug-fitted into the inner bore of the rotor hub 114 by making the outer diametrical dimension of the rotor magnet 120 somewhat larger than the inner diametrical dimension of the rotor hub 114 and then wedging the rotor magnet 120 into the rotor hub 114.
It is consequently the general rule that this so-called outer-rotor type of spindle motor, in which the cylindrical outer surface of the rotor magnets 120 is adhesively fastened to the cylindrical inner surface of the rotor hub 114, is designed so that the separation between the inner diameter of the cylindrical inner surface of the rotor hub 114 and the outer diameter of the cylindrical outer surface of the rotor magnets 120 forms a clearance of several μm. But precisely because the clearance formed is only a few μm, it is difficult to get the amount of adhesive that is applied to be uniform over the entire circumference of the joint. For this reason, if the rotor hub 114 is of short axial and/or radial dimension, stresses produced by hardening and contracting of the adhesive become non-uniform along the circumference, which creates distortion in the joined components. Such distortion is prohibitive of mounting the recording disk(s) on the rotor hub 114 so that the recording face is virtually orthogonal with respect to the center axis of the spindle motor, such that RRO (repeatable runout) worsens.